1. Field of the Invention
The present invention relates to a programmable controller (hereinafter, PLC) system in which a plurality of PLCs are connected via a network.
2. Description of the Related Art
In the case where a plurality of PLCs connected to a network are monitored, a computer (development support device) having a monitoring function is connected directly to a PLC which is desired to be monitored via a communication cable. In such a method, however, the computer should be brought around the PLC which is desired to be monitored each time, and thus this method is inconvenient.
A technique such that a computer is connected only to one PLC on the network and the other PLCs on the network are monitored via the one PLC is, therefore, known. (See, for example, Japanese Patent Application Laid-Open No. 2000-47717.).
With the above publicly-known technique, in the case where the other PLCs on the network (hereinafter, target PLCs) are monitored via the PLC, even if communication cannot take place between the computer and the target PLC, it is quite difficult to find a place on a communication path between the computer and the target PLC where abnormality occurs. Particularly, in the case where the communication path covers a plurality of networks, it is more difficult to uncover the cause for the abnormality.
For example, in a system where a personal computer (development support device) is connected to a specified one PLC (hereinafter, a relay PLC) via a connecting cable and the relay PLC is connected to the target PLC by a network, when a user monitors the target PLC and edits parameters of the target PLC, the following operating steps are taken.
That is to say, at the first step, communication specification (communication protocol, communication speed and the like) between the personal computer and the target PLC is set, and at the second step, the target PLC is online-connected. The target PLC is monitored and its parameters are edited at the third step.
At this time, the connection with the target PLC cannot be made at the second step, and thus an error message such as “communication error” or “type is different” is displayed. The communication is cut during the monitoring of the target PLC at the third step, and thus an error message such as “communication error” is displayed. Such a not-operable condition is occasionally developed.
As the causes for such troubles, the case where a trouble exists between the personal computer and the relay PLC, the case where a trouble exists between the relay PLC and the target PLC, and the case where a trouble exists in the target PLC itself are assumed.
The trouble between the personal computer and the relay PLC includes (1) disconnection of the connecting cable and mis-wiring, (2) an error in communication specification to be set on the side of the personal computer, (3) mis-setting of a communication port of the relay PLC, and the like.
The trouble between the relay PLC and the target PLC includes (1) the disconnection of the connecting cable on the communication path, (2) mis-setting of a routing table previously stored in the PLC on the communication path, (3) communication time-out due to communication load of the network on the communication path, and the like.
The trouble of the target PLC itself includes (1) the case where the target PLC is not an assumed PLC, (2) the case where a communication response is not made due to a change in the setting of the target PLC or reset of the target PLC, and the like.
In the conventional devices, when a PLC is connected via a network, a user can monitor a plurality of PLCs, edit programs and set parameters without switching physical connection of a cable. On the other hand, when communication cannot take place via the network, it takes a very long time to uncover causes for it due to a complicated network path or variety of cause items.
Particularly in a structure of routing which transmits a communication command to the target PLC, when a routing table is previously set in each PLC on the network, a user can connect a target PLC only by specifying a network address and a node address of the target PLC without being conscious of the communication path. In a network where the PLC system is now being constructed, however, the setting of a routing table itself occasionally has a problem. At this time, each setting of the PLCs should be checked one by one, and thus it takes a very long time to solve the problem.